The invention relates to surfactant systems comprising alkoxylated dialkylates. More particularly, the invention concerns the addition of polyethoxylated or polypropoxylated dialkylates to increase the viscosity of surfactant systems and to employ such systems in the enhanced recovery of oil from subterranean formations by surfactant flooding.
In enhanced oil recovery, surfactants are used in micellar and microemulsion floods. By lowering the interfacial tension between reservoir oil and the flood water and connate water, surfactants permit the trapped oil droplets in the reservoir to flow with the surfactant flood water.
Although surfactant flooding is effective in some circumstances in recovering additional oil, it has a number of shortcomings which reduce its value as an economical process. The greatest problem inherently associated with surfactant flooding is poor conformance due to the poor sweep efficiency of surfactant flooding. This is caused by micellar fluids fingering through the reservoir and bypassing large amounts of oil. A principle reason for fingering and poor conformance is the propensity of surfactant systems to move through the reservoir at a faster rate than the displaced oil due in part to the relatively lower viscosity of the surfactant system.
The mobility ratio between the displacing surfactant system and the displaced oil can be improved by viscosifying the surfactant system. A variety of materials have been suggested for incorporation into surfactant and microemulsion systems to increase their viscosity. U.S. Pat. Nos. 3,719,606; 3,827,496; 3,981,361 and others disclose formulations which include polymers as viscosity increasing agents.
Unfortunately, it is nearly impossible to dissolve enough polymer into a micellar or microemulsion system to obtain highly viscous, stable, one-phase systems. Phase separation between the polymer and the micellar phases usually occurs at desirable polymer concentrations. This problem is more acute at salinities typical of most reservoir brines. Salinity reduction may decrease the polymer/surfactant compatibility problem, but usually leads to an increase in the interfacial tension between surfactant and oil. For an effective surfactant or microemulsion process to recover substantial quantities of oil, one needs to have both low interfacial tension and mobility control.
U.S. Pat. No. 4,271,907 teaches one method of incorporating polymers such as polyacrylamides or polysaccharides in a microemulsion system while avoiding some phase separation problems. Greater compatibility with polymers is achieved by incorporating an oil into the microemulsion having an equivalent alkane carbon number (EACN) greater than that of the crude oil for which microemulsion system is designed. Although an improvement, phase separation still occurs and system viscosity cannot be raised to the levels desired.
Once a high EACN microemulsion is injected into a reservoir, it will equilibrate with the environment. As a result, the EACN of the microemulsion oil will decrease due to reservoir crude transporting into the microemulsion. Additionally, the salinity hardness (divalent ion concentration) of the microemulsion will increase as a result of picking up divalent ions from the reservoir matrix. Increased hardness increases phase separation of the polymer from the microemulsion.